it sounds like you have epididymitis or could have just strained an abdominal muscle. It could also be both but you just noticed the testicle part because of your self exam.
I am concerned that you say it is an "ache" and then you say it is "pain." You also say that it is on the epididymus and then your drawing is actually in your abdomen.
I would say that if it does not clear up in a few days (without sex) you should have a doctor examine you.
Vigorous sex will often radiate that "kick in the balls" pain up the spermatic cord into the area you have circled. A covered ice pack on the testicle for several very brief periods (no more than a minute or 2) would help testicular pain and any OTC anti-inflammatory would help. If in a day or so it eases, you probably just strained that area.
However, men are notorious for walking around with this type of thing for months but if it does not heal of its own accord in a few days you may have an infection that needs antibiotic treatment. An infection doesn't only come from a sexually transmitted disease. You can get these from masturbation as well.
The abdominal part really sounds like muscle strain and a covered ice pack on it will help it heal as well.
In either case, if you are having *severe* or *constant* pain, get thee to an ER. An "ache" is not severe and a "kick in the nuts" pain usually passes. It may recur but is not constant. I hope that makes sense. Keep us posted!